A conventional sheet production process, in which the thickness of a sheet such as a macromolecular film is controlled in the transverse direction to have a desired profile such as a uniform thickness, is described below in referent to FIGS. 2 and 3.
A macromolecular polymer as a raw material is extruded from an extruder 3 while being widened in the transverse direction perpendicular to the paper surface of FIG. 2 using a die 4, to form a sheet 1, and the sheet 1 is stretched by a stretching machine 2 in the machine direction (sheet running direction) and the transverse direction (sheet transverse direction), and sheet 1 is wound by a winder 6. The die 4 has plural thickness adjusting means 10 provided at equal intervals in the transverse direction. The thickness adjusting means, for example, heaters or gap adjusters, have function of changing the amounts of the discharged polymer. A thickness gauge 8 measures the thickness distribution of the sheet 1 in the transverse direction of the sheet, and a control means 9 manipulates plural thickness adjusting means 10 based on the measured values at the positions corresponding to the respective thickness adjusting means.
Widely used control means is composed of control loops independently provided for the respective thickness adjusting means, and for each of the control loops, this control means carries out known PID control, in which the result of the proportional-plus-integral-plus-derivative computation of the deviation between a measured thickness value and a target value is delivered as a manipulated variable to each of the thickness adjusting means. Japanese Patent No. 3,021,135 discloses a thickness controller using modern control theory as the thickness control means.
The above-mentioned conventional control system, in which independent control loops are provided for the respective thickness adjusting means, cannot perform sufficiently satisfactory control yet. One of the reasons is that if one of the thickness adjusting means is manipulated, an interference phenomenon occurs in which the sheet thickness vary also at the positions corresponding to the adjacent thickness adjusting means. For this reason, the control loops corresponding to the respective thickness adjusting means interfere with each other, and even if the manipulated variables are computed based on the deviations between the thickness values and the target values of the corresponding positions, for control, it can happen that thickness distribution dose not approach the target values affected by the adjacent adjusting means or that the speed of approaching the target values is very slow.
As another reason, there is a time lag after one of the thickness adjusting means is manipulated till the result is reflected in the thickness result at the corresponding position, that is, a delay time so-called in control occurs. So, if the gain of PID control is made larger, too much manipulation is carried out before the result of the manipulated variable delivered to the thickness adjusting means is reflected in the thickness result at the corresponding position, and the control becomes unstable. Therefore, the gain of control must be kept small to stabilize the control, and the control system is poor in quick responsiveness.
Meanwhile, in the case where, for example, a polyester film is wound as a roll, it can happen that the wound roll is wrinkled or streaked or disfigured at end faces, to extremely lower the value of the produced roll, or even to completely loose the commercial value.
To avoid this problem, it is proposed to improve the surface properties of the film, or to decrease the thickness irregularity, or to disperse the thickness irregularity in the transverse direction of the film by oscillation.
However, the above-mentioned prior art have such a problem that the properties of the film have to be changed, or that the productivity must be lowered, or that the improvement is insufficient. Especially when it is attempted to obtain a thinner film, these problems become more significant.
Furthermore, in recent years, the requirement for better roll form becomes more intensive, and oscillation cannot solve the problem any more.
The invention has been completed to solve these problems. Objects of the invention are to provide a sheet thickness controller that can uniformly and stably control the thickness of a sheet in the transverse direction over the entire width, and to provide a process for producing such a sheet.
Another object of the invention is to provide a roll which form has less wrinkles and streaks, with the productivity sustained at a high level without changing the properties of the sheet.